1. Field of the Invention
The present invention is related to a voltage converter and related driving method, and more particularly, to a voltage converter and related driving method for use in a backlight module.
2. Description of the Prior Art
Light-emitting diodes (LEDs), characterized in low power consumption, long lifetime, high color saturation, fast reaction, anti-quake/pressure ability and small size, have been widely used as backlights in various electronic devices, such as liquid crystal displays (LCDs), scanners, advertising signs or notebook computers. According to actual application, the prior art backlight module normally adopts a white backlight using white LEDs or an RGB backlight using red, green and blue (hereafter as RGB) LEDs.
FIG. 1 is a diagram of a prior art backlight module which includes a DC-DC voltage converter 100 and a backlight 130. The voltage converter 100, including a voltage booster 11 and a pulse width modulation (PWM) circuit 120, is configured to convert an input voltage VIN into an output voltage VOUT for driving the backlight 130. In the backlight 130, white light is generated using white LEDs DW1-DWn and light of other various colors is generated using a color filter. The voltage booster 110 includes an inductor L, a power switch QN, a diode D, resistors R1 and R2, and an output capacitor Co. The power switch QN is configured to control the charging and discharging paths of the inductor L according to a control signal NG: when the power switch QN is turned on, the input voltage VIN charges the inductor L; when the power switch QN is turned off, the energy stored in the inductor L is discharged via the turned-on diode D and transferred to the output capacitor Co, thereby providing the output voltage VOUT for operating the backlight 130. A feedback circuit formed by the resistors R1 and R2 provides a corresponding feedback voltage VFB by voltage-dividing the output voltage VOUT. The boost control circuit 120 is configured to generate the control signal NG according to the feedback voltage VFB: when the output voltage VOUT is too large, the PWM circuit 120 reduces the turn-on time of the power switch QN by adjusting the duty cycle of the control signal NG; when the output voltage VOUT is too small, the PWM circuit 120 increases the turn-on time of the power switch QN by adjusting the duty cycle of the control signal NG. The prior art voltage converter 100 controls the charging and discharging of the inductor L according to variations in the output voltage VOUT, thereby capable of stabilizing the output voltage VOUT. By driving the white backlight 130 using the voltage converter 100, the prior art backlight module is inexpensive and consumes small amount of power, but is unable to provide high quality images due to low color saturation.
FIG. 2 is a diagram of a prior art backlight module which includes a DC-DC voltage converter 200 and a backlight 230. The voltage converter 200, including a voltage booster 110 and a PWM circuit 120, is configured to convert an input voltage VIN into an output voltage VOUT for driving the backlight 230. In the backlight 230, RGB light is generated using red LEDs DR1-DRn, green LEDs DG1-DRn and blue LEDs DB1-DBn, respectively. High quality images can thus be provided by color mixing instead of using a color filter. However, different types of LEDs have different characteristics. For example, the voltage drop of an red LED is normally smaller than that of a blue LED or a green LED. Therefore, the output voltage VOUT of a specific value can not be simultaneously used for displaying multiple colors. Also, the visual effect of images may be downgraded since it requires time to switch between different colors.
FIG. 3 is a diagram of a prior art backlight module which includes a DC-DC voltage converter 300 and a backlight 330. The voltage converter 300, including three voltage boosters 111-113 and three PWM circuit 121-123, is configured to convert an input voltage VIN into three output voltages VOUT1-VOUT3 for respectively driving red LEDs DR1-DRn, green LEDs DG1-DRn blue LEDs DB1-DBn, in the backlight 330. High quality images can thus be provided by color mixing instead of using a color filter. The structures and the operations of the voltage boosters 111-113 and the PWM circuit 121-123 in FIG. 3 are similar to those of the voltage booster 110 and the PWM circuit 120 in FIG. 1. For accommodating different characteristics of the RGB LEDs, the prior art DC-DC voltage converter 300 provides three output voltages VOUT1-VOUT3 using three voltage boosters 111-113. The three inductors L required in the voltage boosters 111-113 occupy large space and increase manufacturing costs.